I am rebuilding the Mikuni BST34 from my 2004 Polaris Sportsman 500HO.
There is a small copper tube (appx. 1/8" OD) that comes out of the casting behind the choke and heads to the "float bowl area?". The tube is completely broken at the choke end although the ends fit together nicely.
What is the purpose of this tube? How might I repair it? What happens if I just leave it as is?
Thanks for the help.

 

Also, it appears I can't remove the diaphragm cover because the screws have rounded heads.
The owners manual says the carb is 1/BST34. The floats are a plastic assembly. I can't find a diagram for my exact model.

 

Also, I find that I cannot remove the screws attaching the diaphragm because there are no removal slots in the heads. I am fighting sputtering at mid range throttle (not at high or low). Should I worry about the diaphragm, or assume it is OK.
I can't find a manual or diagram that looks like my carb with a plastic float assembly.

 

Nobody knows?

 

Sorry, didn't see it needed approval for the pics. Now that that's done, you will probably get a few more replies.

 

This is one those sorry carbs that first appeared on the 2004 Ranger 500. Dirt Cheap Yamaha, Polaris, Suzuki & Kawasaki OEM Parts & Accessories – Cycle Parts Warehouse This is a "non serviceable" carb as no diaphragm parts are listed along with no jet block,slide needle,etc. But this is not rivet but a break off screw head. They can be removed and replaced with regular screws. What it's doing on a Sportsman 500?? Who knows? All this pipe is for is to draw raw fuel through the enrichener circut.These carbs don't have true chokes but the enricheners(plunger) do much the same job. If you can't JB Weld or something similar,cheap knock off BST34 Mikuni carbs are available on ebay. New Polaris Sportsman 500 Carburetor 4x4 ATV Quad Carb 1999 2000 Non HO O | eBay Cheaper than a new 400 buck oem Mikuni.
Your machine should have had the BST40 carb,but sounds like someone did a switcharoo if you have the older black floats on the Bst34. http://www.cyclepartswarehouse.com/f...2004&fveh=5632

 

Update and more questions:
I replaced the needles and seats and other orafices supplied in the rebuild kit except: Did not remove diaphragm cover because of the featureless screws and did not remove the main adjustment screw because ... see pic below.
Reinstalled carb and engine ran at a fast idle for a minute or so then backfired and stalled. After that, it would only run under choke and only for a few seconds. Checking the bowl drain proved gas was in the bowl.
Removed the carb again and in looking at pictures online, saw a pic of one that looked more like mine and it was a BST40. After learning the size difference measurement, I measured mine and it is a BST40, and not a BST34, different than as stated in the owners manual.
With some difficulty, I removed the round head screws on the diaphragm and viewed the diaphragm. See pic below.
Ques:
1. Do you think the needles and seats already replaced are correct? They only said they were for a 2004 Polaris 500HO.http://www.ebay.com/itm/Polaris-Sportsman-500-2003-2005-HO-Carburetor-Carb-Rebuild-Kit-Repair-/181606854625
2. The diaphragm is very misshapen but does not have holes. Does it need replacement?
3. I understand that the main adjustment screw may be covered by a cap. Mine looks like in pic below. Is that a cap to be removed? What should I do to get at the screw for cleaning etc?